The invention relates to fuel injection systems for internal combustion engines in which the fuel injection valves are actuated by electrical fuel control pulses whose duration determines the amount of fuel provided to the engine. The fuel control pulses are generated by an electronic control circuit and their duration is determined primarily on the basis of the instantaneous engine rpm and the air flow rate aspirated by the engine. More particularly, the invention relates to a method and an apparatus for interrupting the fuel control pulses during certain operational phases of the engine, for example during engine braking so as to avoid misfires and exhaust system explosions.
In known apparatus and systems for causing a fuel interruption, a throttle valve transducer senses the position of the throttle flap and when this transducer indicates that the throttle valve opening has fallen below a certain predeterminable value, while the rpm of the engine exceeds a predetermined value, the interruption of the fuel takes place. The throttle valve transducer must follow a sensor function provided by a throttle valve switch, which increases the cost because the throttle angle must be subjected to a correction. A disadvantage of this apparatus is that, in the domain in which the throttle valve is only slightly further open than a predetermined setting of the throttle valve switch and when the rpm has already exceeded the predetermined value, the fuel supply is not interrupted. On the other hand, the probability that the internal combustion engine will already be subject to functional disturbances such as ignition delays, ignition failure, exhaust gas explosions, etc., is related to the flow characteristics of the electromagnetic injection valves and the displacement of the engine. This general domain, which can only be imprecisely defined, makes it very difficult to provide control pulses for the electromagnetic valves whose pulse width is a linear function. This is especially true if the engine has cylinders of small displacement and the aspirated air quantity is relatively small which requires that the amount of fuel delivered by the fuel injection valves be small. In general, when the duration of fuel injection pulses falls below a certain minimum width, it is very difficult to produce the required control components or to operate them and it requires substantial expense to insure proper operation of the engine in this domain of relatively high rpm but very short fuel injection pulses. Overall, there is a multitude of disadvantages which in known fuel injection systems consist, for example, in that a precise control of the fuel quantity as a function of the aspirated air is no longer possible and misfires occur. Again, there may be incomplete or faulty combustion of the mixture which is then exhausted and may contain components which are very detrimental to the exhaust system itself and may also be toxic; especially there may be a high concentration of hydrocarbons.
When the engines are provided with exhaust gas clean-up devices, such as catalyzers, thermoreactors, etc., the increased heating of these devices by misfires is substantially increased, which may result in their destruction.
It has been attempted to counteract these disadvantages by providing vacuum limiters and throttle valve dampers which are actuated by the vacuum in the induction tube of the engine and which affect the quantity of aspirated air. However, these measures do not have the desired result and furthermore the installation of a vacuum limiter has the problem that the engine rpm does not decrease if the throttle is not actuated at the same time. Furthermore, the reliability of these rather expensive additional systems is in doubt.